Characterization of Subclonal Changes Along Progression in Multiple Myeloma.

Clonal composition and clone dynamic changes of neoplasms are a controversial issue, whose investigation is now facilitated by the development of massive parallel sequencing. Here we have analyzed the changes in the mutational spectrum associated with progression, treatment response and relapse in a multiple myeloma patient. We sequenced exomes for the primary quiescent-tumor, the progression and the relapsed samples.

Patient and samples description

Samples from asymptomatic, progression and relapse phases were compared by FISH and Whole exome massive parallel sequencing in a multiple myeloma patient carrying the t(4;14)(p16.3;q32) alteration. At relapse the cytogenetic study identified the presence of two major clones, 13q14 deletion and t(4;14)(p16.3;q32) in the 60% of the cells, and 17p13 deletion in the 12% of the cells.

Whole exome sequencing was performed at CNAG (Barcelona, Spain) following standard protocols for high-throughput paired-end 76pb sequencing on the Illumina HiSeq2000 instruments (Illumina Inc., San Diego, CA). The variant calling was performed using an in house written software calling potential mutations showing a minimum independent multi-aligner evidence.

We performed whole exome sequencing on 3 tumor samples from the same patient: the first one at the time of diagnosis correspond to bone marrow infiltrated by 7% of plasma cells. The two additional samples, at progression and relapse, were done in CD138+ bone marrow cells, at this moment the percentage of infiltration was of 84% and 64% respectively. The germinal DNA from the same patient was used as reference. The mean coverage obtained for the four samples were 93x, with around 85% of bases with at least 20X coverage. After filtering, a total of 104 single nucleotide variations (SNV) were identified, some of them in more than one sample. The variations were classified into silent (25), missense (71), nonsense (6), and essential splice (2), according to their potential functional effect.

In addition to t(4;14) and del13q14, progression and relapse samples shared 36 common SNVs. There were also some variants gained and/or loss in the different time points, suggesting the presence of at least five different clones, independent but related in their evolution. The two main clones were present in progression and relapse samples, but the ratio of the mutant alleles decreased in parallel to the decrement in the percentage of cells carrying on the described cytogenetic alterations

There is a coupling between the cytogenetic and tumor sequence changes indicating that tumor at progression was composed by a dominant clone, together with multiple minor clones. Relapse after treatment was associated with multiple changes in the clone dynamics, progressive reduction of the main clone, emerging of new subclones and lost of minor clones. Dynamic changes along progression could be facilitated/induced by the therapy received.

No relevant conflicts of interest to declare.